Conventionally, a document reading device that irradiates a document with light emitted from a light source and detects light reflected from the document to thereby read the document has been used. There is proposed, as a conventional document reading device, a device provided with a white reference member for performing shading correction of a reading optical system. In this device, the reference member is provided on a glass plate that closes an opening formed in a reading unit for reading a document. The glass plate is slid so that the reference member is located at a position for reflecting light from a light source when a reference value for performing shading correction needs to be obtained and that a portion of the glass plate on which the reference member is not provided and through which light can be transmitted is located at the above-mentioned position when the document reading is performed (see, for example, Patent Document 1).